This invention relates to an encoding device for encoding a digital picture signal into an encoded signal and, more particularly, to an adaptive prediction encoding device.
A digital picture signal is typically an NTSC television signal and comprises successive frames, each frame comprising first and second fields. Each field comprises a plurality of scanning lines. When the digital picture signal is the NTSC television signal, each frame comprises 525 scanning lines.
An encoding device of the type described, comprises a predictive encoder. Responsive to the digital picture signal and a prediction picture signal, the predictive encoder predictively encodes the digital picture signal into the encoded signal by using the prediction picture signal. A local decoder locally decodes the encoded signal into a local decoded signal in response to the prediction picture signal.
A first predictor produces a first prediction signal by delaying the local decoded signal by a first delay equal to a first time duration of two successive scanning lines of the digital picture signal. The first predictor is therefore called a two-previous line predictor in the art. A second predictor produces a second prediction signal by delaying the local decoded signal by a second delay equal to a second time duration of the first field of the digital picture signal when the digital picture signal is the NTSC television signal. The second predictor is therefore named either a previous field predictor or a 262 previous line predictor. A third predictor produces a third prediction signal by delaying the local decoded signal by a third delay less than a third time duration of one scanning line of the digital picture signal. The third predictor is, for example, a higher order intraline predictor.
A comparator compares the first through the third prediction signals with the local decoded signal to produce a control signal. Responsive to the control signal, a selector selects one of the first through the third prediction signals as the prediction picture signal.
The encoding device encodes the digital picture signal into the encoded signal by adaptively switching the first through the third predictors in accordance with the digital picture signal and is therefore called an adaptive prediction encoding device.
The encoding device transmits the encoded signal to a decoding device through a transmission path. When a bit error occurs in a particular picture element of the encoded signal due to the transmission path, the bit error of the particular picture element is propagated to subsequent picture elements which are predictively decoded in the decoding device by using the particular picture element. In order to prevent such propagation of the bit error, refreshment of the encoded signal is carried out in the encoding device.
In order to carry out the refreshment, a conventional encoding device comprises a selection forcing circuit. Responsive to the digital picture signal, the selection forcing circuit forces the selector to select the third prediction signal as the prediction picture signal irrespective of the control signal at the timing of fixed scanning lines which periodically appear at a period of (2N+1) scanning lines in each frame of the digital picture signal, where N represents a natural number. As a result, the refreshment is carried out by forcedly using the third predictor on the fixed scanning lines which periodically appears at the period of (2N+1) scanning lines in each frame of the digital picture signal.
Inasmuch as the natural number N is typically 2, the period (2N+1) of the fixed scanning lines becomes equal to 5 scanning lines. When (2N+1) is equal to 5, the fixed scanning lines correspond to the 1st, the 6th, the 11th, . . . , and the 521st scanning lines of each frame of the digital picture signal.
When N is selected as a small number, the number of the fixed scanning lines increases in each frame. Inasmuch as the third predictor, such as the higher order intraline predictor, is frequently and forcedly used in this case, it is difficult to attain a high encoding efficiency. When N is selected as a large number, the number of the fixed scanning lines decreases in each frame. In this case, it is not possible to effectively refresh the encoded signal.
Thus, the conventional encoding device can not effectively refresh the encoded signal with the encoding efficiency kept high because the refreshment is carried out only on the fixed scanning lines of each frame.